 # Scientific Measurement What is density? From your experimental data, were the densities of the similar objects the same or different? Why? What does this.

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Scientific Measurement What is density? From your experimental data, were the densities of the similar objects the same or different? Why? What does this tell you about density? Can you look up the density of a particular substance? Does the size of the substance play a role in changing its density? For the irregular shaped objects, did you get similar densities for each? Why or why not? If you didn’t, can you give a reason as to why? (accuracy and measuring tools)

Scientific Measurement Qualitative and Quantitative What is the difference between qualitative and quantitative measurements? Qualitative- results that are descriptive and nonnumeric Quantitative- results are given in a definite form, usually as numbers and units Most of the things that we will be doing in chemistry will be quantitative, but there will qualitative elements as well

Scientific Notation (Review) What is scientific notation? Example: 11000000000m s.n. : 1.1 *10 10 m Example: 8.1 *10 -3 m = 0.0081m Example: diameter of a hair: 0.000008m = 8.0*10 -6 m Multiplication 3.0 *10 6 x 2.0*10 3 = (3.0 *2.0) x 10 (6+3) = 6.0*10 9 2.0 *10 -3 x 4.0 *10 5 = 8.0 *10 (-3+5) = 8.0*10 2 (Add exponents) Division 3.0*10 4 /2.0*10 2 = 3.0/2.0 x10 (4-2) = 1.5 *10 2 6.0*10 -2 /2.0*10 4 = 3.0*10 (-2-4) = 3.0*10 -6 (Subtract denominator from the numerator)

Accuracy and Precision, Percent Error Accuracy- measure of how close a measurement comes to the actual or true value of whatever is being measured Precision- measure of how close a series of measurements is to one another Percent error compares the experimental value to the correct value Accepted value- correct value based on reliable references, what types of references, your neighbor? Experimental value- value measured in the lab

Accuracy and Precision

Percent Error Difference between accepted and experimental values is called error Error= accepted value-experimental value % Error= [error]/accepted value * 100% Density of water= 1.0 g/mL (accepted) 0.98 g/mL (experimental) Percent Error = [1.0 g/mL- 0.98g/mL]/1.0 g/mL * 100% = 2%

Significant Figures in Measurements The calibration of your measuring tool determines how many sig. Figs. you can have.

Example #1: This ruler measures to the.1 (in this case centimeters) However, I can see that the measurement lies between the 2.8 and 2.9 measurement, so I can make the estimate that it is approximately 2.83 cm. You see!!! All of those numbers are significant, because they all tell me about the measurement! If I went out any further, it would not be accurate, because my measuring device is not that accurate! Significant Figures in Measurements

This works for other measuring devices as well. Just remember to always go one digit further than the device does Example #1: What temp does the thermometer on the left indicate? The thermometer has whole number digits, so for sig figs I can go to the tenths. The temp is 28.5 o C Significant Figures in Measurements

This also works for Graduated Cylinders Example #1 The drawing above indicates you are looking at a graduated cylinder from the side (note the dip or meniscus, which you always read from the bottom) This graduated cylinder measures to the whole number so we will read it to the tenth This graduated cylinder has a reading of 30.0 ml Significant Figures in Measurements

Rules for Significant Figures Every nonzero digit reported in measurement is assumed to be significant How many sig. Figs.? -24.7m -0.743m -714m three Zeros appearing between nonzero digits are significant How many sig. Figs.? -7003m -40.79m -1.503m Four

Rules for Significant Figures Leftmost zeros appearing in front of nonzero digits are not significant (Act as placeholders) 0.0071m 0.42m 0.000099m two Zeros at the end of a number and to the right of a decimal point are always significant. 43.00m 1.010m 9.000m Four

Rules for Significant Figures Zeros at the rightmost end of a measurement that lie to the left of an understood decimal point are not significant if they serve as placeholders to show the magnitude of the number. 300m (1) 7000m (1) 27210m (4) If 300 was found from careful measurement and not a rough guess, then the zeros would be significant. To avoid this, write in scientific notation. 3x10 2 m - not significant 3.00x10 2 m – significant

Significant Figures in Calculations Calculated values cannot be more precise than the measured values used to obtain it. Addition and Subtraction round to the same number after the decimal place as the measurement with the least number after the decimal place. 12.54m + 349.0m + 8.24m = 369.76m = 369.8m 74.626m – 28.34m = 46.286m = 46.29m

Significant Figures in Calculations Multiplication and Division round answer to the same # of significant figures as the measurement with the least # of significant figures. 7.55m * 0.34m = 2.6 (2) 0.365m * 0.0200m = 0.00730 (3) 2.4526m / 8.4m = 0.29 (2)

SI Units Factor Name Symbol 10 -1 deci d 10 -2 centi c 10 -3 milli m 10 -6 micro µ 10 -9 nano n 10 -12 pico p 10 -15 femto f 10 -18 atto a

SI Units Factor Name Symbol 10 6 mega M 10 3 kilo k 10 2 hecto h 10 1 deka da

Glassware Which are used to measure approximate volumes? Which are used to measure more precise volumes? Which one would you use to measure a large volume, such as 100 mL, accurately?

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